There are two options available:
# Go to Settings, Apps, Google Play Music. You should have “Uninstall” available. Tap on that, it will ask you to confirm. Select YES, and that should be easiest way.
# Same steps as the before, however, if it states “Disable”, that on that, and it will reset the app to factory settings, purge the cache, data and update history.
Google is pushing updates to this app to be allowed to be uninstall, by making it not essential to the system. So, step one should be all you need.
Any questions, let me know.
Alex H.